The Gospel According to St. Luke is the third of the gospels included in the Holy Bible, and describes the life of Jesus Christ, the central figure of Christianity. It is noticeably the longest and most detailed of the synoptic gospels, despite the fact that it derives much of its content from earlier sources. 35% of Luke's content is unique to Luke, as compared to 20% for Matthew and 3% for Mark.
This amount of individuality to Luke's account has lead to two viewpoints concerning its authenticity; the first regards it as the most accurate, and the second as the most fabricated.
From a Christian viewpoint, the Gospel was written by Luke the Evangelist, as decided by the Church Fathers. Luke is mentioned in several of the writings of Paul of Tarsus (see Philemon 1, Colossians 4, 2 Timothy 4, etc.), implying he was a contemporary of the aforementioned. This is supported by the Anti-Marcionite Prologue to the Gospel of Luke, dated to either the second or fourth century:
Luke, a native of Antioch, by profession a physician. He had become a disciple of the apostle Paul and later followed Paul until his martyrdom. Having served the Lord continuously, unmarried and without children, filled with the Holy Spirit he died at the age of 84 years.
Historically, Paul lived roughly around the time of Jesus of Nazareth, though there is no evidence to support that he personally knew him. In fact, Paul describes his conversion to Christianity (he had previously been a Jewish Pharisee, and "violently persecuted" the "Church of God") as occurring when he envisioned the resurrected "Christ Jesus" on the road to Damascus.
Luke's connection to Jesus is only slightly more certain; Epiphanius described him as one of the Seventy Apostles mentioned in the Gospel attributed to him.
Whether Luke wrote the Gospel of Luke or not is debatable, but it can be assumed to be accurate — more so, at least, than with the other New Testament canon.
If we do assume Luke authored this piece, then we can also use its text to better understand the man, and, therefore, his integrity, and the accuracy of his descriptions. We know, for instance, that Luke excludes himself from being involved with Jesus in his life, though they lived at roughly the same time, which seems to disprove the beliefs of Epiphanius, and throw his likelihood to have written a true biographical account into question. Furthermore, the apostle Paul distinguishes between Luke and "brothers of the circumcision", implying Luke was a Gentile, and making his understanding of the Jews less reliable.
Despite this, Sir William Ramsey wrote that "Luke is a historian of the first rank", and E. M. Blaiklock agreed that Luke had "accuracy of detail." Where Luke errors historically is attributed by such men as being due to the "limitations of his sources", and this is the view most taken.
On the other hand, it can also be argued that, as Mark is clearly the first gospel written, dated around 70 CE, the Gospels of Matthew and Luke were derived from Mark to advance claims that Jesus was the Messiah promised in the Old Testament. The organization American Atheists states that Luke and Mark were both unaware of the other's attempts to fabricate the story of Jesus, resulting in such contradictions as the differing genealogies within the two books.
In a nutshell, Luke is one of the most highly esteemed and questioned Biblical books, likely written to convince the reader of the historicity of a divine Christ, and putting Jesus into a historical context with at least adequate success, making it a good source for those researching the early ministry of the Church.